1. Field of the Invention
This invention relates to a ballast and, more particularly, to a ballast that ignites and powers lamps, such as high-intensity-discharge (HID) lamps.
2. Description of the Prior Art
HID lamps include the groups of electrical lights commonly known as mercury vapor, metal halide, high-pressure sodium, and xenon short-arc lamps. Compared to fluorescent and incandescent lamps, HID lamps produce a large quantity of light in a small package.
HID lamps operate by striking an electrical arc during an ignition state and remain turned on to provide lighting during a steady state. The arc is applied across electrodes housed inside a specially designed inner fused quartz or fused alumina tube filled with both gas and metals. The gas aids in starting the lamps during the ignition state. Then, during the steady state, electric power is applied to metals to produce the light once they are heated to a point of evaporation. Like fluorescent lamps, HID lamps use a ballast to ignite and maintain steady state operation.
Known ballasts use electromagnetic induction to provide the proper starting and operating electrical condition to ignite and power the HID lamps. In order to ignite an HID lamp, a relatively high starting voltage of about 25 kV is applied across electrodes of the lamp during the ignition state to place the gases into a suitable ionized condition for striking a glow breakdown. Once ignited, the power applied to the metal terminals of the HID lamp operates it at the warm-up state and steady state to turn on the lamp and provide lighting.
FIG. 1 shows a known ballast for igniting and powering the HID lamp. The ballast receives an input voltage, Vo, of approximately 380 V before ignition of the lamp, and 65 V to 125V after ignition. Typically, the ballast input voltage Vo is provided from a step-up DC-DC voltage converter, such as a flyback converter, that converts a low DC voltage, e.g., 12 V, to the ballast input voltage, Vo. As shown in FIG. 1, the ballast input voltage, Vo, is applied to a switching power inverter comprising a micro-controlled full-bridge DC/AC switching converter that is implemented by switches Q1, Q2, Q3 and Q4 and a full bridge driver. Normally, the power inverter operates at a switching frequency of 100 Hz-500 Hz to avoid acoustic resonance.
A ballast also includes an igniter for generating a high voltage arc based on voltage stored in one or more capacitors. In general, high voltages are desirable for generating the arc since the energy stored in the energy storage capacitor is C·V2/2, where C and V are the capacitance and voltage of the capacitor, respectively. Also higher charge voltages permit a reduction in the capacitor size while maintaining a constant amount of stored energy. In order to provide higher charge voltages, voltage multipliers have been commonly used in the igniter of the ballast.
The ballast of FIG. 1 also includes an igniter comprising a high voltage pulse transformer HV_XFMR that generates a high striking voltage to initiate an ignition arc. The igniter shown in FIG. 1 has a voltage doubler comprising two diodes, D1 and D2, two same sized capacitors, C1 and C2, and two current limiting resistors, R5 and R6. When switches Q1 and Q3 are turned on according to the power inverter's switching frequency, the voltage across terminals A and B becomes positive and capacitor C1 is charged through resistor R5 and diode D1. When switch Q2 and Q4 are turned on at the switching frequency, the voltage across terminals A and B becomes negative and capacitor C2 is charged through resistor R6 and diode D2. Both capacitors C1 and C2 are finally charged to the voltage Vo for a total break-over voltage of 2*Vo. When the voltage across capacitors C1 and C2 reaches the break-over voltage, a spark gap, SG, breaks over and generates a pulse across the primary winding of the pulse transformer HV_XFMR. As a result, a high voltage pulse is generated across the secondary winding igniting the HID lamp.
Once the HID lamp is ignited, the ballast provides required constant power to the HID lamp during its steady state operation at the same switching frequency of the full-bridge DC/AC inverter as the one used to ignite the HID lamp. Immediately after ignition of the HID lamp, a DC or AC warm-up with a switching frequency of several tens of Hz for the power inverter is usually needed to shorten the time to full light output of the HID lamp. During the warm-up interval, the HID lamp is operated with a much higher power. For a 35 W HID lamp, the warm-up power can be as high as 75 W.
The major drawback of the ballast of FIG. 1 is that the effective capacitance of capacitor C1 in series with capacitor C2 is half of the individual capacitance of capacitors C1 and C2, assuming C1=C2. As a result, the utilization of the total energy storage capacity of C1 and C2 is only 50%, which has detrimental effect on the size of the igniter. Another disadvantage is that the firing of the primary winding is not synchronized to the turn on instant of Q1 and Q3, or Q2 and Q4. Therefore, the secondary winding cannot be arranged so that the generated pulse is in phase with the ballast input voltage, Vo, which prevents optimized ignition of the HID lamp.
FIG. 2 shows another prior art ballast disclosed in U.S. Pat. No. 6,437,518. The ballast of FIG. 2 receives a high input voltage of around 380 V from a flyback converter having a transformer winding (12b) and applies it to switching power inverter. The power inverter is a full-bridge DC/AC implemented by switches SW1, SW2, SW3 and SW4 and bridge diodes D13 and D18. When diodes D13 and D18 are forward biased by the voltage across winding (12b), capacitors C16 and C14 are charged and diode 19 is non-conducting. When the voltage across winding (12b) reverses its polarity, capacitor 10 is charged through diode 19, resistor 17, capacitor 16, and switch SW2. Consequently, the voltage across capacitor C10 gradually increases until switch SW11 is closed, and a high voltage pulse is generated to ignite the HID lamp. Under this arrangement, the maximum voltage across capacitor C10 is equal to the sum of the voltage across capacitor C14 and the secondary winding voltage Vin*Np/Ns, where Np and Ns are number of turns of the primary and secondary windings of the flyback transformer, respectively.
The major drawback of ballast of FIG. 2 is that the voltage across capacitor 10 is dependent on the turns ratio Np/Ns of the flyback transformer and ballast input voltage. Also, the voltage across capacitor C10 is usually much lower than twice the voltage across capacitor 14. For example, if Np/Ns=6, Vin=12 V and VC14=380 V, then VC10=380+6·12=452 V, which is less than two times 380 V or 760 V, the necessary voltage for igniting the HID. Therefore, a large capacitor and a pulse transformer with a high turns ratio are required in order to generate a pulse sufficient to ignite the HID lamp. These requirements could lead to significant increase in the size of the ballast.
FIG. 3 shows yet another prior art ballast for automotive high intensity discharge lamps, which is disclosed in U.S. Pat. No. 6,188,180. The ballast of FIG. 3 includes a switching power inverter 10 implemented by switches Q1, Q2, Q3, and Q4 and an igniter 14 implemented by diodes D1 and D2, capacitors C1 and C2 and a resistor R, which form a voltage doubler. The igniter 14 provides the ignition arc to the lamp during the ignition state and a post processing block 12, which controls the switching of switches Q1-Q4 and its frequency, provides the steady state power to turn on the HIP lamp. During steady state operation of the power inverter, Q1 and Q3 are turned on or off while Q2 and Q4 are turned off or on.
When switch Q2 is turned on at the switching frequency of the power inverter 10, capacitor C2 is charged, through the resistor R and diode D2, to a voltage equal to the ballast input voltage across the terminals +V and −V. When Q1 is turned on, again at the switching frequency of the switching inverter 10, capacitor C1 is charged, though diode D1, by the ballast input voltage across terminals +V and −V, plus the voltage across capacitor C2. Consequently, the voltage across C1 is two times the voltage across terminals +V and −V, which is used to generate a pulse at the primary side and ignite the HIP lamp on the secondary side of the transformer T. In the ballast of FIG. 3, the power inverter 10 and igniter 14 operate at the same switching frequency. One drawback of the ballast of FIG. 3, however, is that the igniter has three input connection pins, and the resistor R only limits the charging current flowing to C2, leaving the peak charging current to C1 dependant on the circuit parasitics.
FIG. 4 shows still another prior art ballast described in “Design and analysis of automotive high intensity discharge lamp ballast using micro-controller unit,” IEEE transactions on Power Electronics, pp. 1356-1364, Vol. 18, No. 6, November 2003. The ballast of FIG. 4 has an igniter that uses a stacked winding to boost the voltage. The required DC input voltage for the igniter is obtained using an extra winding of the flyback transformer Tr1. The voltage across a capacitor Cig, which fires an arc gap, is charged by the voltage across capacitors C1 and C2 via a current limiting resistor Rig., where VCig=VC1+VC2. The major drawback of this approach is the requirement for a four-wire connection between the power PC board (PCB) module and an igniter module. Since a high voltage exists in the stacked winding, special care is also needed for the transformer design, PCB layout, and the insulation of the wire connections between the igniter and power circuit, inevitably increasing the cost.
Therefore, there exists a need for a ballast that is small in size and avoids the drawbacks of the prior art approaches.